Concorde's engines at Mach 2
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Concorde's engines at Mach 2
<font face="Verdana, Arial, Helvetica" size="2">We reach a critical point at 1.3 Mach. This is where digitally controlled ramps within the engine intake ducts start to drop down. These ramps create a shock wave that slows the air within the intake. At 1.7 Mach, the ramps are so efficient that we pull the engine out of afterburner and Concorde continues to accelerate. Best of all, fuel flow is cut almost in half. According to Bannister, when cruising at 2.0 Mach, the intakes and exhaust nozzles are producing most of the thrust. The Olympus engines, with the thundering 38,050 pounds of thrust on takeoff, are now producing only nine percent of the total thrust. At 2.0 Mach, the ramps have dropped to almost 45 degrees and the air within the duct slows from approximately 1,350 mph at 2.0 Mach to around 500 mph before it enters the engine. All this wizardry happens in about 11 feet of duct.</font>
Can anybody guess?
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...pressure differential across the duct surfaces .... that which is angled to give a forward vector component yields a thrust ..
Similar reasoning applies to any jet engine in that a lot of the thrust comes from the intake/nacelle leading edge design .... the argument can be carried right through the length of the engine surfaces...
[This message has been edited by john_tullamarine (edited 04 June 2001).]
Similar reasoning applies to any jet engine in that a lot of the thrust comes from the intake/nacelle leading edge design .... the argument can be carried right through the length of the engine surfaces...
[This message has been edited by john_tullamarine (edited 04 June 2001).]
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twistedenginestarter
Some of this ground has been covered recently on another thread. Should you wish to read that thread, then click here.
Briefly, the design of the intake and exhaust systems of a supersonic engine is absolutely critical, and plays a major part in the efficiency of the engine. On Concorde they generate a large percentage of the total thrust required at M 2.0, although not quite as much as reported in the article where it said:
The core engine in fact produces about half the total thrust required at M 2.0.
If I may use part of a recent post by John Farley, quoting from an article by Paul Crickmore on the A12/YF-12/SR71 story, as published in Wings of Fame Vol. 8.
The relevant figures for Concorde’s intakes, at M 2.0 and 60,000 ft, are an engine intake pressure of 8 psi, up from an ambient pressure of 1 psi, with an intake temperature of +125°C, up from an ambient temperature of -56.5°C.
At the exhaust end, the exhaust gas has to be accelerated to very high speed to produce the required thrust, which is done by a system of primary and secondary nozzles forming an efficient convergent/divergent nozzle system.
Bear in mind when considering supersonic airflow that a convergent nozzle will slow and compress a supersonic airflow whereas a divergent nozzle will slow and compress a subsonic airflow.
Under these conditions, approximately 25% of the thrust required is produced by the intake system, another 25% is produced by the exhaust system, leaving the core engine needing to produce only around 50% of the total thrust required in the cruise.
Some of this ground has been covered recently on another thread. Should you wish to read that thread, then click here.
Briefly, the design of the intake and exhaust systems of a supersonic engine is absolutely critical, and plays a major part in the efficiency of the engine. On Concorde they generate a large percentage of the total thrust required at M 2.0, although not quite as much as reported in the article where it said:
<font face="Verdana, Arial, Helvetica" size="2">The Olympus engines, with the thundering 38,050 pounds of thrust on takeoff, are now producing only nine percent of the total thrust.</font>
If I may use part of a recent post by John Farley, quoting from an article by Paul Crickmore on the A12/YF-12/SR71 story, as published in Wings of Fame Vol. 8.
<font face="Verdana, Arial, Helvetica" size="2">The inlet system created internal pressures which reached 18 psi when operating at M 3.2 and 80,000ft, where the ambient pressure is only 0.4psi. This extremely large pressure differential led to a forward thrust vector which resulted in the forward inlet producing 54 per cent of the total thrust. A further 29 per cent was produced by the ejector, while the J58 engine contributed only 17 per cent of the total thrust.</font>
At the exhaust end, the exhaust gas has to be accelerated to very high speed to produce the required thrust, which is done by a system of primary and secondary nozzles forming an efficient convergent/divergent nozzle system.
Bear in mind when considering supersonic airflow that a convergent nozzle will slow and compress a supersonic airflow whereas a divergent nozzle will slow and compress a subsonic airflow.
Under these conditions, approximately 25% of the thrust required is produced by the intake system, another 25% is produced by the exhaust system, leaving the core engine needing to produce only around 50% of the total thrust required in the cruise.
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OK I think I've got it.
Firstly 100% of the thrust comes from the engine. As some of the other commentators have pointed out (eg Checkboard) if you believe otherwise you only need to turn it off to notice the power drops by more than 9 percent or whatever.
This extremely large pressure differential led to a forward thrust vector is just rubbish. It may be true but is a complete red herring.
I think the truth is if you had the engine running at the same altitude but stationary it would produce half the power. The fact that it is moving at Mach 2 doubles the power. Some of this is attributable to the considerable compression of the air entering the engine and some of it is attributable to the efficiencies added by the exhaust geometry.
What this disguises however is that although you are getting say twice as much thrust as you ought to you are having to spend a load of it overcoming the resistance of the forward movement of the engine. I suggest generating a shockwave in an inlet duct is not dissimilar to the work you have to put into the compressor section of a sub-sonic engine.
It is however an intersting point that as the engine struggles by the increasing altitude it is compensated by the increasing speed.
Firstly 100% of the thrust comes from the engine. As some of the other commentators have pointed out (eg Checkboard) if you believe otherwise you only need to turn it off to notice the power drops by more than 9 percent or whatever.
This extremely large pressure differential led to a forward thrust vector is just rubbish. It may be true but is a complete red herring.
I think the truth is if you had the engine running at the same altitude but stationary it would produce half the power. The fact that it is moving at Mach 2 doubles the power. Some of this is attributable to the considerable compression of the air entering the engine and some of it is attributable to the efficiencies added by the exhaust geometry.
What this disguises however is that although you are getting say twice as much thrust as you ought to you are having to spend a load of it overcoming the resistance of the forward movement of the engine. I suggest generating a shockwave in an inlet duct is not dissimilar to the work you have to put into the compressor section of a sub-sonic engine.
It is however an intersting point that as the engine struggles by the increasing altitude it is compensated by the increasing speed.
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twistedenginestarter,
the engine itself does not produce all the thrust. Try to picture the whole assembly without the turbomachinery. What you get is a ramjet which has only a diffuser, a combustion chamber and a nozzle. And, as we all know a ramjet does produce thrust at high speed.
Even better, Philip Hill and Carl Peterson write in their book 'Mechanics and Thermodynamics of Propulsion': At a flight Mach number of 3.5-4, the thermodynamic analysis indicates that the compressor is no longer needed; the most efficient engine is the ramjet.
Isnīt that what happens with the SR-71? Donīt the engines change to ramjets at high speed?
I reckon, you confuse thrust with energy which is put into the engine in form of jet fuel. Think about it, without a supersonic diffuser and converging/diverging nozzle there is no way you would go supersonic in the first place.
the engine itself does not produce all the thrust. Try to picture the whole assembly without the turbomachinery. What you get is a ramjet which has only a diffuser, a combustion chamber and a nozzle. And, as we all know a ramjet does produce thrust at high speed.
Even better, Philip Hill and Carl Peterson write in their book 'Mechanics and Thermodynamics of Propulsion': At a flight Mach number of 3.5-4, the thermodynamic analysis indicates that the compressor is no longer needed; the most efficient engine is the ramjet.
Isnīt that what happens with the SR-71? Donīt the engines change to ramjets at high speed?
I reckon, you confuse thrust with energy which is put into the engine in form of jet fuel. Think about it, without a supersonic diffuser and converging/diverging nozzle there is no way you would go supersonic in the first place.
